In digital integrated circuits, the biggest “control know” available to adjust the amount of power the circuit dissipates is undeniably the supply voltage VDD. Significant energy savings can be realized by lowering the supply voltage until the circuits can just meet the specified performance requirements. We refer to this as dynamic voltage scaling. To accomplish this, what is needed is a very efficient adaptive power supply regulator, preferably one that is small and can be completely integrated on-chip.
Efficient on-chip dc-dc downconversion is also becoming a critical component in the design of deeply scaled digital CMOS ICs. Operating at supply voltages below 1 V, 90-nm (and below) technologies still demand in excess of 100 W of power in the largest chips. Delivering this power at the reduced supply voltage levels required by scaling results in high current requirements, exacerbating power supply integrity issues (i. e., forcing very low impedance requirements on the power distribution). Being able to bring the power onto the chip at higher voltage levels, which are then down-converted to the required supply voltage, significantly reduces the off-chip current requirements. We refer to this as high-tension power delivery.
The most efficient dc-dc voltage converters are buck-type regulators, which generate a reduced dc voltage level by filtering a pulse-width modulated (PWM) signal through a simple LC filter. A buck-type regulator can generate different dc voltage levels by varying the frequency of duty-cycle of the PMW signal. While buck converters can operate at very high efficiencies (>80%), they generally require off-chip filter components, which limits their usefulness for integrated circuit applications.
Two types of on-chip dc-dc converters do not require off-chip components: linear regulators and switched-capacitor power supplies. A linear regulator can be thought of as a variable resistor (implemented as a power FET) that is controlled to produce the required divide-down ratio. As such, its efficiency in generating a voltage Vout from VDD is limited to Vout/VDD. Switched-capacitor (SC) supplies allow one to produce lower voltages at higher efficiencies than linear regulators. SC supplies are effectively capacitance dividers, in which the capacitors are periodically “exchanged” as they are discharged by the load current.
The ideal efficiency of an SC power supply is limited by the amount of “ripple” produced at the output, which can be controlled by the frequency with which the capacitors are switched. The larger the load current, the higher the frequency at which the switched-capacitor supply must be run.
A real switched-capacitor supply suffers additional efficiency degradation due to losses in the switches and overhead associated with generating the clocks and varying their frequency with load. Capacitors with small parasitics capacitances to ground (for example, metal-insulator-metal capacitors) must be used or additional efficiency degradation results. Overall efficiency in generating, for example, a VDD/2 supply voltage is still quite poor (about 60-65%). Furthermore, both linear regulators and switched-capacitor supplies consume huge on-chip areas for both the power transistors (of the linear regulator) and the capacitors (of the switched-capacitor supply).